1. Field of the Invention
The present invention relates to a semiconductor device including an oxide semiconductor and a manufacturing method thereof.
In this specification, a semiconductor device generally means a device which can function by utilizing semiconductor characteristics, and an electrooptic device, a semiconductor circuit, and an electronic appliance are all semiconductor devices.
2. Description of the Related Art
In recent years, attention has been focused on a technique for forming thin film transistors (TFTs) using a thin semiconductor film (with a thickness of from several tens of nanometers to several hundreds of nanometers, approximately) formed over a substrate having an insulating surface. Thin film transistors are widely applied to electronic devices such as an IC and an electro-optical device and have been expected to be rapidly developed particularly as switching elements for an image display device. Various metal oxides are used for a variety of applications.
Some metal oxides have semiconductor characteristics. The examples of such metal oxides having semiconductor characteristics are a tungsten oxide, a tin oxide, an indium oxide, a zinc oxide, and the like. A thin film transistor whose channel formation region includes such metal oxides having semiconductor characteristics is already known (Patent Documents 1 and 2).